The present invention relates generally to the attachment of solder balls to semiconductor packages such as ball grid arrays. More particularly, this invention describes a method and apparatus for the placement of solder balls into a mold prior to attachment to a substrate.
The complexity, functionality, size, and speed of integrated circuit (IC) chips has been increasing as technological advancements allow improvements in all areas of IC use and manufacture. These advancements have also led to increasing numbers of electrical interconnections on the IC chips. The competing forces of smaller size and increased interconnections led to the development of the ball grid array (BGA) and subsequent improvements in that structure have followed continuously since its creation.
In manufacturing semiconductor packages that utilize arrays of solder balls such as BGAs, one of the difficulties encountered is accurately and efficiently aligning and attaching the solder balls to the bottom of the substrate. Various attempts have been made, with differing levels of success, to develop techniques to perform the ball attaching process. Several current methods involve the preliminary step of placing solder balls into a mold or stencil before attaching the balls to the substrate. This preliminary step aids in achieving both quality control objectives and high production requirements.
A typical manual operation uses a fixture consisting of a flat plate having a central cavity within it, approximately the size of the mold, and a raised border around all four sides. A mold is placed in the plate cavity which supports the mold with a minimal gap between the cavity and mold edges. In addition, the plate cavity depth is equal to the height of the mold so that the top surface of the mold is in the same plane as the top surface of the flat plate. Next, loose balls are placed in the fixture. The fixture initially is tilted in one direction approximately 15 to 20 degrees. This allows the loose balls to roll to the low side of the slope. After the fixture is tilted, a vacuum is drawn under the plate cavity and the fixture is tilted in the opposite direction. As the loose balls pass over the mold, empty ball cavities within the mold are filled and the balls are held in place by the pressure differential through the mold cavities. This sequence is repeated until all of the mold cavities are filled with balls. The mold is then removed from the plate cavity and a manual flattening operation is performed.
The manual flattening operation consists of an operator using a hand-held steel roller approximately one inch in diameter. The operator firmly applies the roller over the surface of the mold with the balls seated in the cavities. The mold cavities are configured such that the balls, prior to flattening, extend slightly above the top plane of the mold. The rolling action deforms the balls, which seats the balls firmly into the mold cavities while simultaneously forming a small flat spot on the top of each ball. Moreover, the rolling step causes the balls to be compressed such that the flat spot of each ball is now in the same plane as the top plane of the mold. Thus, the top surface of the balls seated in the cavity and the interstitial surface space of the mold between cavities combine to form a solid, single plane surface. This step completes the process of preparing a mold with balls in a ball grid array format for the subsequent step of applying a metered amount of solder paste to the top of each ball. That step is then followed by attaching the balls to a substrate.
Conventional automated tooling machines for placing balls into a mold utilize methods similar to manual processes. Included in the list of past advancements are the automation of special handling units and vision systems which utilize cameras to detect cavities in the mold which were not properly filled with a solder ball. These methods typically involve two distinct stations. The first station loads the balls into a mold. The mold is then removed from the loading apparatus and, at the second station, the balls are secured in the cavities such that they are all partially flattened and held in place until attached to a substrate, at which time the mold is discarded or reused.
All of the above methods require a two-part process with time spent between steps to move the mold and orient it for the second step of seating the balls. In addition, the first step of typical methods involves random placement of the balls into the mold cavities. This randomness, even if the first step is repeated, often results in vacant cavities. These aspects of the prior manufacturing techniques make them time and labor intensive, which, in turn, makes them more expensive than a method which alleviates the time and labor intensive elements of production.
The deficiencies of the conventional manufacturing techniques show that a need still exists for a method which will accurately and reliably place solder balls into a mold before attachment to a substrate. Therefore, one object of the present invention is to provide a method and automated apparatus to accurately place solder balls into a solder ball mold. Another object of the present invention is to provide a ball flattening step that is nearly simultaneous with the ball placement step, thus shortening the time involved for overall mold preparation.
To achieve these and other objects, and in view of its purposes, the present invention provides a loading method and apparatus for automatically loading preform solder balls into molds in preparation for attachment to a substrate. This invention also includes a nearly simultaneous step that flattens the balls once placed in the mold. This flattening step is performed as the apparatus used in the first step of placing the balls into the mold cavities is being reset, and does not require the mold to be moved from the position it occupied during the first step. Automating an accurate ball placement apparatus and additionally providing a flattening step which does not require time to be spent reorienting the mold after ball placement will improve the cost effectiveness of solder ball loading molds.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.